The escalating requirements for high density and performance associated with ultra large scale integration impose correspondingly high demands on photolithographic techniques.
Conventional photolithographic techniques utilize a photoresist, i.e., a polymeric composition wherein a developer solvent will selectively remove only the exposed (or, for different compositions, selectively only the unexposed) portions of the photoresist. This leaves a patterned photoresist layer in place which provides a patterned mask for subsequent steps such as ion implementation, etching, or patterned deposition of materials by lift-off techniques (i.e., depositing a material over all and then removing the remaining portions of photoresist to leave the material only where the photoresist was not present). Since critical dimensions on the semiconductor device are predetermined by the dimensions of the openings in the photoresist curing mask, it essential that each step in the photolithography process transfer an accurate patterned mask for each subsequent step, i.e, that critical dimensions are maintained throughout the photolithography process.
A problem with conventional photolithography is pattern degradation resulting from the reflection of light from the layer being patterned. Anti-reflective coatings have been used in an attempt to solve this problem. Anti-reflective coatings are designed, by appropriate adjustment of variables such as composition, deposition conditions, and reaction conditions, to exhibit the requisite optical parameters to suppress multiple interference effects caused by the interference of light rays propagating in the same direction due to multiple reflections in the photoresist film. The effective use of an anti-reflective coating enables patterning and alignment without disturbance caused by such multiple interference, thereby improving line width accuracy and alignment, which are critical factors with respect to achieving fine patterns with minimal spacing. For example, the use of an anti-reflective coating is particularly significant when forming a via or contact hole over a stepped area, as when etching a dielectric layer deposited on a gate electrode and gate oxide formed on a semiconductor substrate in manufacturing a field effect transistor.
Typically, anti-reflective coatings are spun onto the wafer surface and a photoresist is then spun on top of the anti-reflective coating. After masking, the photoresist is cured and the wafer developed by means of wet chemical etching to remove the uncured (or cured, depending on photoresist) portions of the photoresist and those portions of the anti-reflective coating lying beneath the uncured (or cured, depending on photoresist) photoresist. In order to maintain critical dimensions, the development step should completely remove all portions of the anti-reflective coating lying beneath the uncured (or cured, depending on photoresist) photoresist. In other words, the pattern in the anti-reflective coating should accurately reflect the pattern in the photoresist after the development step. When there is this identity in pattern between the photoresist and the anti-reflective coating after the development step, the anti-reflective coating becomes an accurate mask for the patterning of subsequent layers and thus, critical dimensions in the resulting fabricated semiconductor device are maintained.
However, this identity in pattern between the photoresist and anti-reflective coating after the development step is not always realized due to the formation of a "foot" on the anti-reflective coating. Although the manifestation of a "foot" on anti-reflective coatings is well-known and various theories exist as to their cause, the "foot" abnormality has not, to date, been fully understood. Nonetheless, these "footings" narrow the opening in the photoresist through which the anti-reflective material is to be removed and removal of the anti-reflective material through this opening results in an anti-reflective layer which is an inaccurate mask for subsequent layers. Thus, the formation of a "footing" changes the critical dimensions in the resulting fabricated semiconductor device.
Therefore, in the fabrication of semiconductor devices, there exists a need for an improved photolithography technique whereby the beneficial effects of an anti-reflective coating may be realized while maintaining critical dimensions in each subsequent step through accurate transfer of pattern masks.